1. Field of the Invention
The present invention relates to an output power detection circuit of a transmitter and, more particularly, to an output power detection circuit having no temperature dependency.
2. Description of the Related Art
An output power detection circuit of a conventional transmitter will be described with reference to FIG. 2. A transmission signal is amplified by a driver amplifier 31 and is further amplified by a power amplifier 32 to a predetermined output power, and the resultant signal is supplied to an isolator 33. The signal is transmitted from an antenna (not shown) via an antenna sharing device (not shown).
A part of the transmission signal from the power amplifier 32 is supplied to an output power detection circuit (hereinafter, called a detection circuit) 34 where a voltage (detection voltage) proportional to the output power is detected.
The detection voltage is amplified by a dc amplifier 35 and is converted into a digital signal by an A/D converter 36. The digital signal is inputted to a not-illustrated control circuit, properlyprocessed, and outputted as a control signal. The control signal controls the amplification degree of the driver amplifier 31 to make the output power from the power amplifier 32 constant.
The detection circuit 34 has two detection diodes 42 and 43 to which a bias current in the forward direction is applied by a voltage from a power source terminal 41. The cathode of the detection diode 42 and the anode of the other detection diode 43 are connected, so that the two detection diodes 42 and 43 are connected in series. The cathode of the detection diode 43 is connected to the ground via voltage dividing resistors 44 and 45 which are connected in series.
A part of the transmission signal from the power amplifier 32 is supplied to a connection point between the cathode of the detection diode 42 and the anode of the detection diode 43 via a coupling capacitor 46 and a series resistor 47. The anode of the detection diode 42 is connected to the ground via a direct current blocking capacitor 48 and the cathode of the detection diode 43 is connected to the ground via a smoothing capacitor 49.
In the above construction, when a part of the transmission signal from the power amplifier 32 is supplied to the detection circuit 34, it is rectified by the two detection diodes 42 and 43 and a detection voltage proportional to the output power is obtained from the connection point between the voltage dividing resistors 44 and 45.
In the above construction, since the static characteristics of the detection diodes 42 and 43 have temperature dependency, when the temperature around the detection diodes 42 and 43 changes, the detection voltage is changed accordingly. Consequently, there is a problem that an accurate output electric power cannot be detected.
That is, for instance, when the ambient temperature rises, the static characteristic of the detection diodes 42 and 43 changes in such a manner that the forward current increases, thereby increasing the bias current. As a result, the detection voltage appearing at the connection point of the voltage dividing resistors 44 and 45 also increases. The detection voltage increases by the increase in the bias current flowing in the detection diodes 42 and 43, so it is not the output power which is accurately detected. For example, when the amplification degree of the driver amplifier 31 is controlled on the basis of the detection voltage, a predetermined output power cannot be therefore obtained.
It is therefore an object of the invention to provide an output power detection circuit of a transmitter, which can accurately detect an output power from a power amplifier without depending on a change in ambient temperature.
In order to realize the object, an output power detection circuit of a transmitter of the invention comprises: a first diode to which a first load resistor is connected in series and through which a first bias direct current flows; a second diode to which a second load resistor is connected in series and through which a second bias direct current flows; and a differential amplifier in which a first voltage generated in the first load resistor is applied to one of input terminals and a second voltage generated in the second load resistor is applied to the other input terminal. It is set so that the first voltage generated in the first load resistor by the first bias direct current and the second voltage generated in the second load resistor by the second bias direct current are made equal. By adding a part of a transmission signal outputted from the power amplifier to the first diode, a voltage according to the power of the transmission signal is detected from the differential amplifier.
According to an output power detection circuit of a transmitter of the invention, the first and second diodes have static characteristics which are the same, the first and second load resistors are set to have resistance values which are equal, and the first and second bias direct currents are applied from the same power source.
According to an output power detection circuit of a transmitter of the invention, at least one of the first and second load resistors is constructed by including a variable resistor.
According to an output power detection circuit of a transmitter of the invention, each of the first and second diodes has two diodes which are connected in series, a transmission signal from the power amplifier is inputted to the connection point of the two diodes which construct the first diode, and the transmission signal is rectified to a double voltage by the two diodes constructing the first diode.